A conventional centrifugal compressor is known as means for compressing air. The patent literatures listed later disclose inventions relating to centrifugal compressors. A centrifugal compressor is used in a supercharger of an internal combustion engine, in particular, a turbocharger.
A typical conventional supercharger of an internal combustion engine uses a compressor configured as shown in FIG. 14. The compressor has an outer shell, which is formed by a housing 102 and a back plate 106. The back plate 106 is fixed to a bearing housing (not shown), and the back plate 106 and the housing 102 are fastened to each other with a bolt.
A shroud 104 is formed in the housing 102, and an impeller 110 is housed in the shroud 104. The impeller 110 has a hub 112 supported by a bearing (not shown) so as to be rotatable about a rotational axis CL, and a plurality of blades 114 attached to a surface of the hub 112.
An annular diffuser 120 is provided around the periphery of the impeller 110 so as to surround the impeller 110. The diffuser 120 is formed by a shroud-side wall 124, which is a part of the housing 102, and a hub-side wall 122, which is a part of the back plate 106. The shroud-side wall 124 is seamlessly connected to a surface of the shroud 104, and the hub-side wall 122 is connected to the surface of the hub 112 via a step formed by the outer edge of the hub 112. With the compressor of the typical conventional supercharger, the shroud-side wall 124 and the hub-side wall 122 are each formed as a flat surface perpendicular to the rotational axis CL of the impeller 110. Although the diffuser 120 illustrated in FIG. 14 is a vaneless diffuser, which has no vane, the supercharger of the typical conventional internal combustion engine may use a compressor provided with a vane diffuser, which has a vane.
In the housing 102, a spiral scroll 130 is provided around the periphery of the diffuser 120 so as to surround the diffuser 120. Air taken in by the compressor is accelerated by the rotating impeller 110 and then decelerated by the diffuser 120 and thereby compressed. The compressed air flowing from all around the perimeter of the diffuser 120 is collected by the scroll 130, and the resulting one flow of air is fed to a downstream inlet pipe.
A problem with the internal combustion engine provided with a supercharger is deposit on the inner wall of the compressor. The deposit grows from oil mist contained in blow-by gas. With the internal combustion engine for a vehicle, the blow-by gas leaking from the combustion chamber to the crankcase is fed back to the inlet channel and processed there. In the case of the internal combustion engine provided with a supercharger, the blow-by gas is fed back to upstream of the compressor in the inlet channel. The oil mist in the blow-by gas contains carbon soot resulting from combustion of fuel, and the oil mist adhering to the wall of the compressor is increased in viscosity and turned into deposit in the high temperature atmosphere. The deposit in the compressor decreases the efficiency of the compressor and therefore degrades the performance of the internal combustion engine.
With the conventional compressor configured as shown in FIG. 14, in particular, deposit on the hub-side wall 122 of the diffuser 120 poses a problem. FIG. 15 schematically shows a flow of oil mist in the diffuser 120 of the conventional compressor. The oil mist is conveyed by the flow of compressed air ejected from the impeller 110 in a direction that is not in parallel with the walls 122 and 124 of the diffuser 120. In the longitudinal cross section including the rotational axis CL of the impeller 110, the walls 122 and 124 of the diffuser 120 are in parallel with a line L1 that is perpendicular to the rotational axis CL of the impeller. Since the compressed air ejected from the impeller 110 still partially flows in the axial direction, however, the direction of the flow of the oil mist is inclined toward the hub-side wall 122 from the perpendicular line L1. As a result, a large amount of oil mist collides with and adheres to the hub-side wall 122. The oil mist has a high surface area to volume ratio and therefore quickly evaporates, so that the oil mist is increased in viscosity immediately after the oil mist adheres to the hub-side wall 122, and is turned into deposit on the hub-side wall 122.
To the contrary, less deposit is formed on the shroud-side wall 124. This is because a smaller amount of oil mist adheres to the shroud-side wall 124 due to the direction of the flow, and oil flowing to the shroud-side wall 124 along the surface of the shroud 104 prevents growth of the deposit on the shroud-side wall 124. From these considerations, it can be said that it is important to reduce the amount of deposit on the walls of the diffuser, in particular, the hub-side wall, in order to reduce the amount of deposit in the compressor and maintain the efficiency of the compressor.